The present invention relates to hydrostatic transmissions, and more particularly, to such transmissions of the closed-loop type.
Although the present invention may be utilized with hydrostatic transmissions in which the pump and motor elements may be of various types, the invention is especially advantageous when used in a hydrostatic transmission in which the pump and motor are of the radial piston (ball) type. Therefore, the invention will be described in connection with an embodiment of the radial ball type.
A typical hydrostatic transmission of the radial ball type includes a pintle assembly having journal portions on which the radial ball elements are rotatably mounted. The journal portions define grooves which provide fluid communication between the radial ball elements and passageways defined by the journal portions, the passageways comprising the high pressure and low pressure system conduits.
A hydrostatic transmission of the type to which the present invention is especially suited is produced by the assignee of the present invention and sold commercially under the designations Model 7 and Model 11. Such a transmission is illustrated and described in greater detail in U.S. Pat. No. 4,091,717, assigned to the assignee of the present invention and incorporated herein by reference. Such transmissions are typically used to propel lawn and garden tractors, or other similar vehicles which are relatively small and inexpensive. Thus, the size, weight, and expense of the transmission must be minimized to be commercially acceptable for use on such vehicles.
Although such hydrostatic transmissions have been extremely successful commercially, there have occasionally been pressure-related failures of various components. For example, there have been instances where pressure spikes resulted in the breaking of the journal portion at the groove which communicates fluid to or from one of the radial ball units. In addition, there have also been instances where pressure spikes (or torque spikes) have resulted in broken output shafts.
In order to overcome such pressure related failures, it would appear obvious to one skilled in the art to provide pressure relief valves. However, in hydrostatic transmissions of the type to which the present invention relates, it would not be economically feasible to incorporate a pair of commercially available relief valve cartridges or relief valve assemblies. Furthermore, aside from the cost, there is the problem of the location of such relief valves. For example, in the commercially available Model 7 and Model 11 transmissions referred to previously, the pintle (disposed between the radial ball pump and radial ball motor units) already includes a pair of damper pistons (to overcome the effects of vibrations emanating from the variable radial ball pump); a pair of check valve assemblies (to insure that neither system conduit is below reservoir pressure); and a pair of acceleration valve assemblies (to insure a smooth start-up of the vehicle, from a stop condition, for either forward or reverse movement). Therefor, it would be nearly impossible, as a practical matter, to add relief valve assemblies without substantial redesign of at least the pintle assembly.